The invention disclosed herein relates generally to a technique and implementation for preventing undesired interconductor coupling and spurious signals in a switching matrix, and more particularly to apparatus and a technique for eliminating cross talk, electrical interference and sensing ambiguities in a capacitive keyboard.
Electronic keyboards have long comprised a primary form of input device for information handling and data processing systems. With the proliferation of and technological advances in such systems has come increased demands on the performance and durability of keyboard input devices. Further, the rapid recent proliferation of smaller and less expensive systems requiring keyboard input devices has increased the demand for less expensive keyboards.
Capacitive keyboards are well known in the field of data input devices. They provide certain inherent advantages over electrical contact keyboards. These advantages particularly include mechanical simplicity, long life and absence of electrical signal problems caused by contact bounce and corrosion.
However, as set forth in numerous publications, including U.S. Pat. No. 3,750,113 issued to J. Cencel on July 31, 1973, and U.S. Pat. No. 3,921,166 issued to J. Volpe on Nov. 18, 1975, capacitive keyboards have suffered from another problem which is particularly significant in capacitively coupled systems. This problem stems from the fact that there is inherent capacitive coupling between every pair of elements in a keyboard. Thus, in a typical capacitive keyboard having a matrix of conductors including drive signal and sense signal conductors, a signal on any conductor will normally appear to some extent on every other conductor. Obviously, this phenomenon complicates the task of detecting which of an array of variable capacitors between pairs of conductors in the matrix is actuated, thus varying the capacitive coupling it provides.
As noted in the above-identified patents, a variety of techniques have been employed in an attempt to minimize problems caused by stray capacitive coupling. These techniques include the use of ground lines interleaved between the drive and/or sense conductors, ground planes and various forms of shielding. In addition, various electronic signal detection, verification and processing techniques have been employed to improve the reliability of detecting and distinguishing valid key actuation signals from signals caused by stray coupling. Some techniques representative of this approach are disclosed in U.S. Pat. No. 3,931,610 issued to R. Marin, et al on Jan. 6, 1976, U.S. Pat. No. 4,163,222 issued to D. Gove on July 31, 1979 and U.S. Pat. No. 4,211,915 issued to D. Miller, et al on July 8, 1980, as well as previously identified U.S. Pat. No. 3,921,166.
All of the foregoing techniques involve the addition of elements to either the basic keyboard matrix or the signal detection, verification and processing circuitry. Accordingly, these techniques are undesirable to the extent that they contribute to the complexity, size, cost and potential unreliability of the keyboard systems.
In addition, all of these techniques suffer to some extent from an inability to effectively eliminate the basic problem of stray coupling and/or its effects on signal detection. Specifically, although screening lines, ground planes and shielding do provide a degree of isolation of individual conductors in the keyboard matrix, such isolation is not complete and stray coupling is not totally eliminated. With or without the use of conventional screening and shielding, electronic signal detection and verification is only effective to the extent that signals due to stray coupling and other electrical interference differ from valid signals and to the extent that such differences can be feasibly detected.
The present invention avoids many of the principal disadvantages of the above-described techniques through the use of electronic grounding of inactive conductors in the basic keyboard matrix. Electronic grounding is simply accomplished with a minimal addition of common electronic components. No elements are required to be added to the basic keyboard matrix. Stray coupling is effectively totally eliminated. Finally, the present invention inherently provides N-key rollover operation.